In general, a vehicular transmission comprises a speed change mechanism and a clutching mechanism (also referred to as "a starting clutch"). The speed change mechanism includes an input member, which is driven by an engine, and an output member, which is connected to wheels of a vehicle. The starting clutch controls the mechanical power transmission for starting, driving and stopping the vehicle in relation with the engagement and disengagement of the clutch. In general, the engagement of the clutch is controlled hydraulically, and in most clutching mechanisms, the hydraulic control for the operation of the clutch is carried out, for example, in relation to the rotational speed of the engine, which is detected by a sensor, and an electromagnetic valve is actuated in correspondence with the rotational speed detected.
To avoid a failure in the hydraulic control when an electrical breakdown (e.g., a breakdown of the electrical control system) or an open slick (i.e., the condition where the spool of a valve sticks in its open state) occurs, such hydraulic controllers are often equipped with a backup mechanism, in which a signal pressure is generated in correspondence with the rotational speed of the engine, and the hydraulic control is carried out by using this signal pressure.
As such a backup mechanism, a mechanism which uses a pitot tube is known and described as an example of prior art in Japanese Patent Application No. H10(1998)-57827, which was applied by the same applicant as this patent application. FIG. 5 shows a starting clutch controller of prior art which uses a pitot tube. In this hydraulic circuit, a pitot flange 51 is provided in a direct connection to the output shaft of the engine, and a pitot tube 53 is provided against the rotational direction in the lubrication oil which accumulates in the oil chamber of the pitot flange 51. The pitot tube 53 extracts the dynamic pressure of the lubrication oil, which moves in the pitot flange along with the rotation of the engine, as a signal pressure which corresponds to the rotational speed of the engine. In the following paragraph, this hydraulic circuit used for controlling the starting clutch is described in detail.
In this controller, the pressure of the hydraulic oil supplied from a tank T by a pump P is adjusted to a line pressure PL by regulator valves 71 and 72, and this line pressure PL is supplied to a starting clutch control value 81. In the starting clutch control valve, a control pressure is produced in correspondence with the electrical current applied to the linear solenoid 81a of this valve. This control pressure is supplied through an accumulator 86 and a shift restriction valve 82, which are provided on a line 92 to the starting clutch 5, to control the engagement of the clutch.
On the other hand, the line pressure PL is also supplied through another line 91c to a shift control valve 85, and a shift control pressure is produced in correspondence with the electrical current applied to the linear solenoid 85a of this valve. This shift control pressure is supplied through a line 94 to a shift restriction valve 82 as a signal pressure. The shift control pressure is used as the pressure for hydraulic control of the widths of the drive and driven pulleys of the belt-type continuous speed change mechanism, which is not shown in the drawing.
As shown in FIG. 6 in detail, the shift restriction valve 82 includes a spool 82a, which is biased rightward by a spring 82b, and the shift control pressure creates a thrust which pushes the spool 82a leftward. The position of the spool 82a in the shift restriction valve 82 is controllable in correspondence with the magnitude of the shift control pressure, so this pressure is able to control the switching of the circuits for the starting clutch, the adjustment of the valve opening, etc.
This controller is designed such that if it is determined that there is a breakdown in the circuits of the hydraulic controller, for example, the starting clutch control valve 81 or the electrical circuit breaks down, then the current applied to the shift control valve 85 is turned OFF to fix the shift control valve 85 to its open condition. As a result, the line 94 receives the same line pressure as the line 91. FIG. 6 shows all the valves in their breakdown mode.
In this mode, the spool 82a of the shift restriction valve 82 is pushed leftward against the spring 82b by the line pressure which is applied through the line 94. As shown in the drawing, the line 92 and the line 93, which had been in fluid communication, are now closed to each other while the line 91b and the line 95 are in fluid communication with each other. In this condition, the line pressure in the line 91b acts on a pitot regulator valve 83 through a line 95, and portion of the hydraulic oil is supplied through a line 95b that is branched from the line 95 to an oil supply pipe 52, from which the oil is discharged into the oil chamber of the pitot flange 51.
The hydraulic oil 54 which accumulates in the oil chamber of the pitot flange 51 is rotated along with the pitot flange 51 in correspondence with its viscosity as the pitot flange 51 is directly connected to the output shaft of the engine. The detector portion at the tip of the pitot tube 53 is positioned in this hydraulic oil in the direction against the rotation of the flange. Thus, the pitot tube 53 detects the dynamic pressure which is proportional to the flow speed of the hydraulic oil, which is rotating. As a result, the pressure (signal pressure) which corresponds to the rotational speed of the engine is supplied from the pitot tube 53 through a line 96 to the pitot regulator valve 83.
The pitot regulator valve 83 includes a spool 83a, which is biased leftward in the drawing by a spring 83b, and the above mentioned signal pressure (also referred to as "the pitot pressure" in the following) which corresponds to the rotational speed of the engine generates a thrust rightward for the spool. Thus, the position of the spool 83a is controlled in correspondence with the rotational speed of the engine. Therefore, the pitot regulator valve 83 regulates the line pressure in the line 95 in correspondence with the pitot pressure, i.e., the signal pressure, and supplies the pressure which corresponds to the rotational speed of the engine, through the lines 97 and 93 to the starting clutch 5.
In this clutch control circuit, the pitot regulator valve 83 substantially closes the fluid communication between the lines 95 and 97 while the rotational speed of the engine is relatively low, for example, when the vehicle is in idling condition. In this condition, the clutch is in the disengaged condition, so the vehicle will not start. When the rotational speed of the engine is increased a little, i.e., the accelerator pedal is stepped down a little., this valve opens partially, so the vehicle will start gradually. When the accelerator pedal is stepped further downward to increase the rotational speed of the engine, the clutch is engaged completely, and the vehicle is able to travel continuously.
By the provision of such a backup mechanism, the vehicle can start and stop gradually even when the hydraulic controller for the clutch experiences a failure because of, for example, a breakdown in the hydraulic circuit of the hydraulic controller including the starting clutch or a breakdown in the electrical control system.
However, in this type of backup mechanism, the gain adjustment of the clutch control pressure (i.e., the adjustment of the clutch control pressure against the signal pressure) involves the adjustment of not only the: spool of the starting clutch control valve 81 but also that of the shift restriction valve 82. Also, the gain adjustment of the clutch pressure for the abnormal condition requires the adjustment of the two spools which are in the shift restriction valve 82 and the pitot regulator valve 83. Therefore, a number of man-hours must be spent for adjusting and setting the backup mechanism to keep an appropriate balance in these valves both for the normally operating condition and for the abnormal condition.
In addition, in this hydraulic circuit, the clutch control pressure, which has been already adjusted for controlling the clutch, must pass through an additional valve (e.g., the shift restriction valve 82 in the drawing) in either normal or abnormal condition. Therefore, there is a concern that the existence of such a valve increases the possibility of oil leak and resultant pressure loss especially around this valve and the oil passages connected to it (e.g., the lines 92, 95 and 97).
Moreover, this hydraulic circuit of prior art is constructed such that the clutch control pressure, which actuates the starting clutch, passes through the starting clutch control valve 81 (in the normal condition) and the pitot regulator valve 83 (in the abnormal condition). As a result, for example, if the engine is replaced with another one which has a larger output power, and therefore the capacity of the clutch must be increased, then these valves must be also replaced to increase the capacities as well as the capacity of the linear solenoid valve 81a. This is another disadvantage.